1. Field of the Invention
The present invention relates to a voice communications system adaptive for broadband communications, and more particularly to gateway apparatus capable of communicating from a telephone set selectively with an Internet Protocol (IP) network or to a public switched telephone network (PSTN).
2. Description of the Background Art
The Voice over IP (VoIP) technique has recently come into widespread use due to the lower network or communication cost established by assembling voice signals into IP packets to thereby integrate voice with data. On the one hand, the traditional public switched telephone network (PSTN) attaches importance to however voice signals are to be transmitted, and transmission band is limited to 3.1 kHz or less. The network design architecture restricts the bandwidth for a unit channel to 3.1 kHz, while digital transmission networks have a basic transmission rate of 64 Kb/s with the sampling rate of 8 KHz. This means that both the terminals and the networks have handled signal frequencies exceeding 3.1 kHz as unnecessary signals.
On the other hand, with recent tendency towards broadband services, the transmission equipment on the networks is designed to cope with broadband communications. Moreover, even the central office lines are being adapted to broadband networks by means of asymmetrical digital central office lines (ADSL) or optical transmission lines, such that end-to-end broadband voice transmissions have become available. As a result, high-quality voice communications have become a desideratum.
However, gateway equipment and telephone subscriber sets compatible with the VoIP technology, which recently have become popular, merely attaches importance to coping with VoIP, but have been unable to realize high-quality voice communications which have exploited the advantages of the broadband networks. Although the broadband telephone set, which widens the frequency band of the voice signals, has been proposed, this type of the telephone set, if connected to the public switched telephone network, violates its technical conditions.
In an application where the general subscriber telephone sets, dedicated to the public switched telephone network, are connected over a gateway unit to the broadband transmission channel, if in-channel signals, such as voice signals, transmitted between the general subscriber telephone sets and the gateway unit are subjected to signal conversion for coping with broadband communications, it would be difficult to provide for sufficient speech quality because of the natures specific to the general subscriber telephone sets.
Moreover, when designing a certain telephone set, it is neither definite whether the telephone set will be connected to the public switched telephone network or to the IP network nor whether the telephone set will be connected directly or via a gateway unit to the network. Even a telephone set designed to cope with high-quality or wide-band transmission, when interconnected to the system, such as a gateway unit, would often prevent signals from being transmitted appropriately. Although it may be contemplated to correct the characteristics of a telephone set, when installed in the system, it is not definite which conditions are to be used for the correction, and it may be feared that the telephone set, even when corrected, fails to achieve its optimum performance. Although it may also be contemplated to compensate for the characteristics of the telephone set by the function of the gateway unit, to which is connected the telephone set, it is not possible to sufficiently compensate for some of the characteristics, such as acoustic ones, of the telephone set that cannot be comprehended from the gateway unit.
In light of the above, such a system has been desired which exploits the features of the broadband network to achieve high-quality broadband voice communications, or which is able to cope with voice communications over the public switched telephone network while coping with the broadband technique.